British Prime Minister Gordon Brown said Wednesday that airlines flying empty planes simply to maintain valuable airport slots were "unacceptable" due to their environmental impact.

His comments came after a newspaper report claimed that Britain's third-biggest airline was planning to operate such so-called "ghost flights" to conserve slots at Heathrow, London's main airport.

"It would be clearly unacceptable if airlines flew without any passengers whatsoever simply to maintain their slots," Brown said when asked about the report at his weekly question session in the House of Commons.

"We shall look at the matter that you raise, we will talk to the airlines," he added.

The Times newspaper reported that the airline bmi plans to operate nearly empty flights during the forthcoming winter months to save its Heathrow slots, valued at 770 million pounds -- a claim rejected by the airline.

A bmi spokesman said the airline "categorically denies that it will operate empty or near empty services at Heathrow this winter in order to protect its slots at the airport."

The paper also said another airline, Flybe, has advertised for actors to fly between the eastern English city of Norwich and Dublin to meet passenger targets.

The practice of flying planes with few or no passengers has been reported before, but the issue is more in the spotlight now due to concerns about climate change and soaring fuel prices.

A spokesman for the opposition Liberal Democrats slammed such flights. "Given the massive carbon emissions for which aviation is responsible, isn't this an environmental disgrace?" Norman Baker asked Brown in parliament.